This invention relates to reconfigurable tunnel field-effect transistors and apparatus employing such transistors.
Tunnel field-effect transistors (TFETs) are based on a pin diode or p-n diode operated in the reverse bias region. The reverse bias current exploited in these transistors is a tunneling current arising due to quantum mechanical tunneling of carriers between energy bands of source and channel regions of the TFET structure. An n-type TFET (N-TFET) has a p-type source region and an n-type drain region separated by a channel region which has an overlying gate. A p-type TFET (P-TFET) has an n-type source region and a p-type drain region separated by the gated channel region. In each case, the structure is such that application of an appropriate gate voltage to the channel region causes a shift in the energy bands of the structure, enabling band-to-band tunneling to occur between the source and channel regions.
Since operation is based on band-to-band tunneling, TFETs can operate at significantly lower power than the MOSFETs (metal-oxide semiconductor field-effect transistors) employed in CMOS (complementary metal-oxide semiconductor) technology. Low-power operation is increasingly important as technology is scaled down and device dimensions are reduced. However, unlike the symmetrical n-p-n or p-n-p structure of MOSFETs, the TFET structure is inherently asymmetric. This structural asymmetry means that bidirectional current conduction is not feasible in TFETs as illustrated in FIG. 1 of the accompanying drawing. This figure shows an illustrative example of the asymmetric output characteristic of an N-TFET due to the inherent diode between source and drain. The reverse bias tunneling current does not in principle present a threshold in a tunnel diode. In forward bias, however, the diode structure presents a threshold for the onset of the on-current, usually at a gate voltage in the range of 0.3-0.5V. Below this threshold the diode will only be able to conduct a very small current. Hence, for low values of gate bias, TFETs will conduct substantially higher currents in reverse than in forward bias. Since TFETs are targeted for low-power operation, voltages are expected to be in the sub-0.5V range to present an advantage over CMOS, i.e., below the turn-on voltage in the forward direction of the diode. Symmetrical bi-directional operation is therefore not possible.
Bidirectional transistor operation is a fundamental requirement in various circuit applications. One example is pass-gate applications where the transistor is used to selectively pass an input logic level to the output in either direction of current flow. The conventional SRAM (static random access memory) cell provides a common example of apparatus which employs pass gates. The SRAM cell comprises a pair of cross-coupled inverters connected between two signal lines by respective access transistors. The access transistors serve as pass-gates for controlling flow of current into and out of the cell during read and write operations. The asymmetry of TFETs means that they are unsuitable for use as the access transistors in this cell structure. While SRAM designs particularly suited for TFETs have been proposed, these designs involve significantly increased complexity, requiring additional transistors and/or more complex circuitry for controlling cell operation.
Doping of semiconductors to produce semiconductor devices is conventionally achieved via chemical implantation to introduce impurities into the intrinsic semiconductor material. However, electrostatic doping has also been employed in some devices. Electrostatic doping involves application of a voltage to a gate overlying a semiconductor to control carrier concentration in a region of the semiconductor beneath the gate. For example, electrostatic doping of silicon nanowires and carbon nanotubes to form p-n junctions is described in: International Patent Application publication no. WO 2009/128777 A1; “Carbon nanotube p-n junction diodes”, Lee et al., Applied Physics Letters, 2004, vol. 85, pp. 145-147; and “Transport in carbon nanotube p-i-n diodes”, Bosnick et al., Applied Physics Letters, 2006, 89, 163121. “Reconfigurable Silicon Nanowire Transistors”, Heinzig et al., Nano Letters, 2012, 12, p. 119-124, describes use of electrostatic doping to program the channel of a silicon nanowire FET to either n-type or p-type, permitting dynamic configuration of either an nFET or a pFET. A first gate is used to control channel polarity and a second gate is used to control carrier concentration.